Active differential and motor vehicle

ABSTRACT

An active differential for the controlled distribution of a drive torque generated by a drive motor to two drive shafts includes a planetary gear train configured to couple the two drive shafts to a drive shaft of the drive motor, and a distributor motor including a distributor shaft. The distributor motor produces a torque, with a distribution of a drive torque to the two drive shafts being dependant on the torque produced by the distributor motor. The distributor shaft and the planetary gear train are coupled by a coupling device which only transmits a torque from the planetary gear train to the distributor shaft when a rotational speed difference between rotational speeds of the two output shafts exceeds a predetermined limit value and when a connection condition depending on an operating condition of the distributor motor is satisfied.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is the U.S. National Stage of International ApplicationNo. PCT/EP2014/003021, filed Nov. 11, 2014, which designated the UnitedStates and has been published as International Publication No, WO2015/078560 and which claims the priority of German Patent Application,Serial No. 10 2013 019 906.9, filed Nov. 28, 2013, pursuant to 35 U.S.C.119(a)-(d).

BACKGROUND OF THE INVENTION

The invention relates to an active differential for the controlleddistribution of a drive torque generated by a drive motor to two outputshafts, including a planetary gear train for coupling the output shaftsto a drive shaft of the drive motor and a distributor shaft to adistributor motor, wherein the distribution of drive torque to theoutput shafts depends on a torque exerted by the distributor motor.

Active differentials are used to distribute drive torques to several, inparticular two, output shafts. The differential thus allows the twooutput shafts to have different rotational speeds. When the differentialis operated without an additional distributor motor or a complete orpartial locking of the differential, the same torques are transmitted toboth output shafts. Active intervention in a differential which changesthe torque distribution to the output shafts depending on a furtherintroduced torque allows a flexible distribution of torques.

Active differentials are used in particular in motor vehicles for thedistribution of the drive torque from a drive motor to the driven wheelsof the motor vehicle. In the automotive segment, the use of such anactive differential is often referred to as “torque vectoring” or“active yaw”. The use of an active differential in the motor vehicleallows in particular an active influencing of the yaw angle of the motorvehicle, since the torques to the individual wheels and thus the forcesbeing transmitted to the respective wheel on the roadway can beseparately controlled or adjusted by the active differential.

In active differentials, even very small differences in speed betweenthe output shafts lead to rotation of the distributor shaft and thus thedistributor motor. Since a relatively small-sized motor is typically tobe used as a distributor motor, a translation between the rotationalspeed difference of the drive axles and the distributor shaft istypically provided in order to be able to generate large torquedifferences between the drive axles with the relatively small-scaledistributor motor. Therefore, small differences in rotational speedbetween drive axles can already lead to a continuous rotation of thedistributor motor with a medium rotational speed. This can lead tosteady charging and discharging of the battery with low currents,additional mechanical stresses and in particular an uneven load ofclocked power electronics. Thus, both the electrical and the mechanicalcomponents of the active differential are already stressed at minimumrotational speed differences.

In particular, when an active differential is used in a motor vehicle,however, such strains can occur over long periods, such as when thetires of the motor vehicle are worn unevenly and thus when driving thereis always a small difference in rotational speed between the drive axleswhich are coupled with the tires, or when the weight shift in the motorvehicle is uneven. Even very small steering movements, as they aretypically continuously performed by drivers, lead to continual strain onthe active differential.

SUMMARY OF THE INVENTION

The invention is thus based on the object of specifying an activedifferential, whose electrical and mechanical components are lessstressed by small differences in rotational speed between the driveaxles at a low load.

The object of the invention is solved by an active differential of theaforementioned type, wherein the distributor shaft and the planetarygear train are coupled by a coupling device that only transmits a torquefrom the planetary gear train to the distributor shaft when therotational speed difference between the rotational speeds of the outputshafts exceeds a predetermined limit value, as well as complying with aconnection condition that depends on an operating state of thedistributor motor.

The invention is based on the idea that the initially describedtransmission of small rotational speed differences between the outputshafts to the distributor motor is disadvantageous and it is thereforeadvantageous to prevent speed-dependent torque transmission, at least inthe direction of the distributor motor. In the simplest case, thecoupling device can completely decouple the distributor shaft from theplanetary gear train when the rotational speed difference between therotational speeds of the output shafts is below a specified limit value.This is particularly possible since the rotation of the distributorshaft in the coupled state is performed with a rotational speed thatcorresponds to the rotational speed difference between the drive axlesscaled with a translation factor. The rotational speed of thedistributor shaft itself in the coupled state, or the rotational speedof a shaft which couples the planetary gear train to the couplingdevice, can thus be regarded as a quantity that is proportional to therotational speed difference. It can therefore be considered as dependenton the rotational speed difference as well as on this proportionalrotational speed. In the simplest case, the coupling device may berealized by a coupling which couples the elements to be coupled only ata predetermined minimum rotational speed, for example, as acorrespondingly designed centrifugal clutch. Of course, it is alsopossible to measure the rotational speed difference or the rotationalspeed proportional to the rotational speed difference by a sensor, tocompare to a limit value using a switching or a control device and tocorrespondingly control a controllable, particularly electricallycontrollable coupling device.

It should hereby be possible to also control the active differential byapplying a torque through the distributor motor in the cases when therotational speed difference between the drive axles is slight. Forexample, it is possible when using the differential in the motorvehicle, that a higher torque will be transmitted specifically to one ofthe wheels when starting up or stopping on the mountain or when drivingover an obstacle such as a curb or similar in order to overcome theobstacle or to compensate for a not fully horizontal roadway, which turnwheels of the motor vehicle and thus the drive axles however withidentical or nearly identical rotational speed. To control thedifferential by a torque of the distributor motor, however, a torquefrom the distributor shaft on the planetary gear train has to betransmitted. However, in doing so, a counter-torque is transmitted fromthe planetary gear train to the distributor shaft. Therefore, it isadvantageous that the coupling device according to the inventiontransmits torque in a manner independent of rotational speed from theplanetary gear train to the distributor shaft when complying with theconnection condition that depends on the operating state of thedistributor motor. The connection condition can, in this way, especiallyevaluate an energization of the distributor motor and/or a control ofthe distributor motor for generating a torque.

This is easily possible when a controllable coupling device is involvedthat is controlled by a control device. The control device can basicallycontrol the coupling device for coupling the planetary gear train to thedistributor shaft, when there is a situation in which a torque is to betransmitted deliberately. Basically, however, it is also possible toform the coupling device mechanically in such a way that a torquetransmission between distributor shaft and planetary gear train ispossible when falling below a rotational speed difference duringoperation of the distributor motor for generating a torque.

The coupling device may in particular be a centrifugal clutch or includea centrifugal clutch. A centrifugal clutch can be easily designed sothat a coupling between the planetary gear train and the distributorshaft then takes place exactly when a predetermined rotational speed ofthe planetary gear train or a connecting shaft between planetary geartrain and coupling device is exceeded. Since the connecting shaft has arotational speed which is proportional to the rotational speeddifference between the rotational speeds of the output shafts, acentrifugal clutch can be used in order to couple a planetary gear trainto a distributor shaft only when a predetermined rotational speeddifference is exceeded.

Alternatively, the coupling device may be a viscous clutch or include aviscous clutch. In that way, the viscous clutch can have a definedtorque closure from a predetermined minimum rotational speed andtransmit no or minimal torque at low rotational speeds. This applies forthe viscous clutch as for the centrifugal clutch mentioned above.

It is advantageous when the coupling device includes a locking device,wherein the coupling device is configured for the rotationalspeed-independent transmission of torque from the planetary gear trainto the distributor shaft and from the distributor shaft to the planetarygear train with active locking device. In doing so, the differential canin particular include a control device which is configured to activatethe locking device when complying with the connecting condition thatdepends on an operating state of distributor motor. In doing so, theoperating state, which leads to an activation of the locking device, inparticular, can be an energization of the distributor motor, wherein acurrent limit value is provided in particular, at whose crossing overthe locking device is activated. However, it is also possible thatinformation is provided for the control device, on which the control ofthe distributor motor is also based. It is thus possible, for example,that a motor vehicle system for control of the distributor motor forgenerating a defined torque also has the torque target value of thecontrol device so that this is activated when controlling thedistributor motor for generating the torque or when exceeding apredetermined limit value by which the torque target value can activatethe locking device. Thus, it is possible, for example, to energize amagnet with the energization of the motor, which activates the lockingdevice, or to activate the locking device depending on the torquesacting in the coupling device.

Alternatively or in addition to the use of elements of the couplingdevice, which simply mechanically induce or separate a coupling of theplanetary gear train and distributor shaft depending on the rotationalspeed difference, it is possible to use especially electricallycontrolled elements of the coupling device. It is advantageous when thedifferential includes a sensor for detecting the rotational speeddifference and a control device, wherein the coupling device includes acoupling element that in a connected state, transmits a torque from aplanetary gear train to the distributor shaft and from the distributorshaft to the planetary gear train, and does not transmit a torque in adisconnected state, wherein the control device is configured to controlthe coupling element depending on the rotational speed difference forswitching between the connected state and the disconnected state.

The sensor may be arranged in particular on a shaft whose rotationalspeed is proportional to the rotational speed differential, especiallyto the distributor shaft or a shaft which leads from the planetary geartrain to the coupling device. The use of a sensor and a control devicehas in particular the advantage that a very flexible control of thecoupling element is possible. With this flexible control, one can easilyachieve, especially even at low rotational speed differentials, thepossibility of transmitting torque from the distributor shaft to theplanetary gear train. In addition, it is easily possible to adjust thepredetermined limit when using a control device. In the differentialaccording to invention, the torque transmission should be prevented fromthe planetary gear train to the distributor shaft particularly forcontinual, yet small rotational speed differences between the rotationalspeeds of the drive shafts. In particular, for use in a motor vehicle,it is advantageous to configure this limit variable since, for example,the rotational speed differences caused by various retractions of thetires may change over a certain time. Therefore, the predetermined limitvalue can be adjusted for use in a control device of the coupling devicedepending on changes of the average rotational speed differences overlonger time periods.

The control device may hereby be configured in particular for thepresence of a connection condition that depends on a operating state ofthe distributor motor, regardless of the rotational speed difference, tocontrol the coupling element for the change into and for the remainingin the connected state. The connection condition can, in this way,particularly evaluate the operating state of the distributor motor. Theevaluation of the operating state of the distributor motor can performedas explained above for the activation of a locking device.

The coupling elements can be designed in various ways. The couplingelement may thus be a magnetic coupling, wherein at least one magnet ofthe magnetic coupling can be controlled by the control device forswitching between the connected and the disconnected state. Magneticcouplings can be controlled in which the position of a disk or thearrangement of a magnetic powder or a magnetic fluid is changed byswitching a magnet, so that, depending on the switching of theelectromagnet, the coupling element connects or disconnects thedistributor shaft and the planetary gear train. By using a magneticcoupling, a particularly simply designed electrically controllablecoupling element can be formed.

Alternatively, the coupling element may include at least one couplingactuator, which can be controlled by the control device for switchingbetween the connected and disconnected states and which is designed tomove at least one mechanical element of the coupling element. Dependingon the specific requirements for the coupling device in the activedifferential, such a controllable mechanical coupling can beadvantageous as a coupling element. In this case, the coupling actuatorcan especially move the disks of a disk coupling.

As explained above, a torque transmission can then in particular beachieved only in one direction at low engine speeds, when the couplingdevice includes a freewheel. Therefore, the coupling device may includeat least a freewheel coupling the distributor shaft and the planetarygear train and a locking element associated to the freewheel, wherein alocking position of the locking element is locked to the freewheel and afree rotation of the distributor shaft in relation to the planetary geartrain is possible in an open position of the locking element in apredetermined rotational direction, and wherein the position of thelocking element depends on the rotational speed difference. The positionof the locking element can be changed, in particular by an actuator,which is controlled by a control device. Alternatively, however, thelocking element may also be mechanically designed such that it is moveddepending on rotational speed between an open position of the lockingelement and a locking position. For example, a centrifugal clutch or aviscous clutch, each with an additional catch, can be used so that, forexample, a freewheel is reached in a separation of the centrifugalclutch, and a torque transmission in both directions is possible with aconnection of the centrifugal clutch.

Since, typically, a torque transmission from the distributor motor tothe planetary gear train should be possible in both directions, inparticular two freewheels which have opposite locking directions can beprovided in the coupling element, wherein the locking elements areseparately controllable, in particular by a control device.Alternatively, however, the locking elements can also be designed andarranged such that they are moved depending on the rotational speed androtational direction between the locking position and the open positionas a result of the mechanical interaction effects in the couplingdevice.

Furthermore, the invention relates to a motor vehicle including adifferential according to one of the embodiments described above. Asexplained above, it is possible in particular when using an activedifferential in the motor vehicle, that small differences in rotationalspeed continuously occur between the wheels and thus between the driveaxles and thus the mechanical and electrical components of the activedifferential are unnecessarily stressed. It is therefore advantageous touse a differential according to the invention, which transmits a torquefrom the planetary gear train to the distributor shaft only when therotational speed difference between the rotational speeds of the driveshafts exceeds a predetermined limit value as well as when complyingwith a connection condition that depends on a operating state of thedistributor motor.

The output shafts may in particular each be coupled to a wheel of themotor vehicle.

BRIEF DESCRIPTION OF THE DRAWING

Further advantages and details of the invention will become apparentfrom the following embodiments and the accompanying drawings. They show:

FIG. 1 schematically an embodiment of the active differential accordingto the invention,

FIG. 2 schematically a further embodiment of the active differentialaccording to the invention,

FIG. 3 the coupling device of a third embodiment of the activedifferential according to the invention, and

FIG. 4 an embodiment of the motor vehicle according to the invention,

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an embodiment of an active differential 1 for thecontrolled distribution of a drive torque generated by a drive motor totwo output shafts 3, 4. The coupling of the drive motor 2 and the driveshaft to the output shafts 3, 4 takes place through the planetary geartrain 5 of the differential 1. In addition to the planetary gear train5, the differential 1 includes a distributor motor 6, wherein thedistribution of drive torque to the output shafts 3, 4 depends on thetorque exerted by the distributor motor 6. The distributor motor 6 iscoupled via a distributor shaft to the coupling device 7, which in turnis coupled to the planetary gear train 5. In a complete decoupling ofthe distributor shaft from the planetary gear train 5 by the couplingdevice 7, the differential has 1 the behavior of a non-lockeddifferential, so that the torque generated by the drive motor 2 isequally distributed to the output shafts 3 and 4. When the distributorshaft coming from the distributor motor 6 is connected to the planetarygear train 5 by the coupling device 5, the torque of the distributormotor 6 thus controls the distribution of the drive torque to the outputshafts 3 and 4.

For large rotational speed differences between the output shafts 3 and4, the coupling device 7 should couple the distributor shaft with theplanetary gear train 5. When the rotational speed difference between therotational speeds of the output shafts 3 and 4, is small however, thecoupling device 7 should decouple the distributor shaft at least suchfrom the planetary gear train 5 so that no torque from the planetarygear train 5 can be transmitted to the distributor shaft and thus to thedistributor motor 6. To achieve this, the coupling device 7 is designedsuch that it transmits a torque from the planetary gear train 5 to thedistributor shaft only when the rotational speed difference between therotational speeds of the output shafts 3, 4, exceeds a predeterminedlimit value as well as energization of the distributor motor 8. Thisutilizes that, due to the coupling of the output shafts 3, 4 to aconnecting shaft which connects the planetary gear train 5 to thecoupling device 7, this connection shaft has a rotational speed whichcorresponds to the rotational speed difference between the output shafts3 and 4 scaled with a translation factor of the planetary gear train 5.A limit value for the rotational speed difference between the rotationalspeeds of the output shafts 3, 4 corresponds to a derived limit valuefor the rotational speed of the connecting shaft.

Since the connecting shaft is coupled directly to the coupling device 7,a coupling device can be used as a coupling device 7, which induces arotational speed-dependent coupling of the connecting shaft to thedistributor shaft. The desired function of the coupling device isconcretely achieved through this so that the coupling device is acentrifugal clutch. The centrifugal clutch is designed such that at lowrotational speeds, the connecting shaft is not coupled to thedistributor shaft, but after having reached a certain minimum rotationalspeed, a coupling of connecting shaft and distributor shaft occurs andtherefore of planetary gear train 5 and distributor motor 6. In doingthis, a magnet is additionally provided on the centrifugal clutch, whichis also energized for energization of the distributor motor 6 and whenenergized the centrifugal clutch connects regardless of rotationalspeed.

An advantageous development of the active differential 1 shown in FIG. 1is shown in FIG. 2. The active differential 8 is substantially designedas differential 1 shown in FIG. 1, wherein it differs from differential1 shown in FIG. 1 by the design of the coupling device 14 and by theadditional use of a sensor 15 and a control device 16. Like differential1 shown in FIG. 1, a drive torque of the drive motor 9 is distributed tothe output shafts 10 and 11 in the embodiment according to FIG. 2 by theplanetary gear train 12 of the differential 8. Here, the distributormotor 13 is coupled to the planetary gear train 12 via the distributorshaft and the coupling device 14. A sensor is additionally arrangedbetween intermediate planetary gear train 12 and coupling device 14,which detects the rotational speed of the distributor shaft 12 andcoupling device 14. The data detected by the sensor 15 are madeavailable to the control device 16. The control device 16 therebycontrols the drive motor 9, the distributor motor 13 and the couplingdevice 14 depending on the rotational speed detected by sensor 15 and inaddition data present in the motor vehicle.

The coupling device 14 is hereby configured as a magnetic coupling whichconnects or does not connect the distributor shaft to the planetary geartrain 12 depending on a control signal of the control device 16. Thecontrol device 16 detects the rotational speed detected by the sensor 15for controlling the coupling device. Since the control device 16 alsocontrols the distributor motor 13, information about the target torqueis also present in the control device 16, which is to be exerted by thedistributor motor 13. The control of the coupling device 14, thus theenergizing or the non-energizing of the electromagnet of the magneticcoupling, occurs depending both on the target torque for the distributormotor 13 and the rotational speed of the distributor shaft detected bythe sensor 15. When the distributor motor 13 is controlled in order togenerate a target torque, the coupling device 15 is always controlled inorder to connect the connecting shaft and the distributor shaft.However, when there is no control of the distributor motor 13 forgenerating a torque, control of the coupling device 14 takes placedepending on the rotational speed of the distributor shaft detected bythe sensor 15. When the detected rotational speed is lower than or equalto a threshold value, the coupling device 14 is controlled in order toseparate the distributor shaft and the connecting shaft and thus theplanetary gear train 12 and the distributor motor 13. When therotational speed exceeds the limit value, the coupling device 14 iscontrolled by the control device 16 in order to connect the connectingshaft and the distributor shaft and thus the planetary gear train 12 andthe distributor motor 13. It is therefore possible with the activedifferential 8 independent of rotational speed to transmit torque fromthe distributor shaft and thus the distributor motor 13 to the planetarygear train 12 and thus to actively affect the distribution of the torqueof the drive motor 9 to the output shafts 10, 11. At the same time,however, a transmission of torque to the distributor motor 13 isprevented due to low rotational speed differences.

In an alternative, not separately shown embodiment of an activedifferential, it is also possible for the coupling device to bemechanically configured such that a torque transmission only takesplace, when the connection shaft has a certain minimum rotational speed,i.e. designed, for example, as a centrifugal clutch, in addition,however a locking device is provided on the coupling device, which onactivation, leads to a rotational speed-independent transmission oftorque from the distributor shaft to the planetary gear train. Such alocking device may be designed as an actuator which actuates a lockingpin. In this case, the additional sensor can be omitted, since therotational speed-dependent control of the coupling device isaccomplished solely by the mechanical structure of the coupling device.In this case, the locking device of the coupling device can be activatedby the control device if and when the distributor motor is controlled togenerate torque. Thus the same function of the active differential isachieved as that of the differential 8 as above-described with respectto FIG. 2.

FIG. 3 shows a further possible embodiment of a coupling device of anactive differential. The coupling device 17 couples the connecting shaft18 which connects the coupling device 17 and the not-shown planetarygear train to the distributor shaft 19 which connects the couplingdevice 17 to the not-shown distributor motor. The coupling device 17thereby includes two freewheels 20, 21 which each have an opposingfreewheeling direction. Without additional elements, this would meanthat a free rotation of the connecting shaft 18 in relation to thedistributor shaft 19 is possible at all times. Therefore, the freewheels20, 21 include additionally locking elements which lock the freewheelsin a locking position and lead to a tight coupling of the shafts of thefreewheel in both directions. The locking elements are each separate bythe control device 23 from an open position in which a free rotation inthe rotation direction predetermined for the respective freewheel 20, 21is possible, able to be brought into a locking position in which thefreewheel is locked. When a rotational speed is detected by therotational speed sensor 22 arranged on the connecting shaft 18 which areabove a predetermined limit, the control device 23 controls the lockingelements in such a way that the freewheels 20 and 21 are locked, thatis, a torque transmission in both directions is possible. When therotational speed detected with the sensor 22 however, is below the limitvalue, and a torque is additionally to be transmitted from the not-showndistributor motor to the not-shown planetary gear train, the lockingelement of one of the freewheels 20, 21 is controlled by control device23 in such a way that it is brought into the open position and thelocking element of the other of the freewheels 20, 21 is controlled insuch way that it is brought into the locking position. A torquetransmission from distributor motor to the planetary gear train in apredetermined direction of rotation is thus possible, but smalldifferences in rotational speed between the drive axles can also befiltered in a rotation direction when using the distributor motor todistribute the torque of the drive motor to the drive axles.

FIG. 4 shows an embodiment of a motor vehicle which includes an activedifferential. The motor vehicle 24 has a rear wheel drive, wherein therear wheels 25, 26 are operated by the drive motor 27. The distributionof the drive torque provided by the drive motor 27 to the wheels 25, 26is performed by the active differential 28. The active differential 28is constructed in accordance with the differential 1 shown in FIG. 1.Alternatively, a construction of the differential 28 in accordance withthe differential 8 shown in FIG. 2 or according to one of the otherexplained embodiments would also be possible of course.

What is claimed is:
 1. An active differential for the controlleddistribution of a drive torque generated by a drive motor to two driveshafts, comprising: a planetary gear train configured to couple the twodrive shafts to a drive shaft of the drive motor; a distributor motorincluding a distributor shaft, said distributor motor producing atorque, with a distribution of a drive torque to the two drive shaftsbeing dependant on the torque produced by the distributor motor; and acoupling device coupling the distributor shaft and the planetary geartrain and configured to only transmit a torque from the planetary geartrain to the distributor shaft when a rotational speed differencebetween rotational speeds of the two output shafts exceeds apredetermined limit value and when a connection condition depending onan operating condition of the distributor motor is satisfied.
 2. Theactive differential of claim 1, wherein the coupling device is acentrifugal clutch or includes a centrifugal clutch.
 3. The activedifferential of claim 1, wherein the coupling device is a viscous clutchor includes a viscous clutch.
 4. The active differential of claim 1,wherein the coupling device includes a locking device and is configuredto effect a rotation-speed-independent transmission of torque from theplanetary gear train to the distributor shaft and from the distributorshaft to the planetary gear train, when the locking device is activated.5. The active differential of claim 4, further comprising a controldevice configured to activate the locking device, when the connectioncondition that depends on the operating condition of the distributormotor is satisfied.
 6. The active differential of claim 1, furthercomprising a sensor configured to detect the rotational speeddifference, and a control device, said coupling device including acoupling element configured to transmit a torque in a connected statebetween the distributor shaft and the planetary gear train, from theplanetary gear train to the distributor shaft and from the distributorshaft to the planetary gear train, and to be prevented in a disconnectedstate between the distributor shaft and the planetary gear fromtransmitting a torque, said control device being configured to controlthe coupling element such as to change between the connected state andthe disconnected state in dependence on the rotational speed difference.7. The active differential of claim 6, wherein the control device isconfigured to control the coupling element for changing to the connectedstate and to remain in the connected state in the presence of aconnection state that depends an operating state of the distributormotor, regardless of the rotational speed difference.
 8. The activedifferential of claim 6, wherein the coupling element is a magneticclutch including at least one magnet controlled by the control devicefor changing between the connected and disconnected states.
 9. Theactive differential of claim 6, wherein the coupling element includes atleast one coupling actuator controlled by the control device forchanging between the connected and disconnected states and configured tomove at least one mechanical element of the coupling element.
 10. Theactive differential of claim 1, wherein the coupling device includes atleast one freewheel to couple the distributor shaft and the planetarygear train, and a locking element operably connected with the freewheeland movable in dependence on the rotational speed difference between alocked position in which the freewheel is locked and an open position inwhich the distributor shaft is able to freely rotate in relation to theplanetary gear train to a predetermined relative rotational direction.11. A motor vehicle, comprising: two drive shafts; a drive motorgenerating a drive torque; and an active differential for distributingthe drive torque to the two drive shafts, said active differentialincluding a planetary gear train configured to couple the two driveshafts to a drive shaft of the drive motor, a distributor motorincluding a distributor shaft, said distributor motor producing atorque, with a distribution of a drive torque to the two drive shaftsbeing dependant on the torque produced by the distributor motor, and acoupling device coupling the distributor shaft and the planetary geartrain and configured to only transmit a torque from the planetary geartrain to the distributor shaft when a rotational speed differencebetween rotational speeds of the two output shafts exceeds apredetermined limit value and when a connection condition depending onan operating condition of the distributor motor is satisfied.
 12. Themotor vehicle of claim 11, wherein each of the two drive shafts arecoupled with a wheel of the motor vehicle.
 13. The motor vehicle ofclaim 11, wherein the coupling device is a centrifugal clutch orincludes a centrifugal clutch.
 14. The motor vehicle of claim 11,wherein the coupling device is a viscous clutch or includes a viscousclutch.
 15. The motor vehicle of claim 11, wherein the coupling deviceincludes a locking device and is configured to effect arotation-speed-independent transmission of torque from the planetarygear train to the distributor shaft and from the distributor shaft tothe planetary gear train, when the locking device is activated.
 16. Themotor vehicle of claim 15, further comprising a control deviceconfigured to activate the locking device, when the connection conditionthat depends on the operating condition of the distributor motor issatisfied.
 17. The motor vehicle of claim 11, further comprising asensor configured to detect the rotational speed difference, and acontrol device, said coupling device including a coupling elementconfigured to transmit a torque in a connected state between thedistributor shaft and the planetary gear train, from the planetary geartrain to the distributor shaft and from the distributor shaft to theplanetary gear train, and to be prevented in a disconnected statebetween the distributor shaft and the planetary gear from transmitting atorque, said control device being configured to control the couplingelement such as to change between the connected state and thedisconnected state in dependence on the rotational speed difference. 18.The motor vehicle of claim 17, wherein the control device is configuredto control the coupling element for changing to the connected state andto remain in the connected state in the presence of a connection statethat depends an operating state of the distributor motor, regardless ofthe rotational speed difference.
 19. The motor vehicle of claim 17,wherein the coupling element is a magnetic clutch including at least onemagnet controlled by the control device for changing between theconnected and disconnected states.
 20. The motor vehicle of claim 17,wherein the coupling element includes at least one coupling actuatorcontrolled by the control device for changing between the connected anddisconnected states and configured to move at least one mechanicalelement of the coupling element.
 21. The motor vehicle of claim 11,wherein the coupling device includes at least one freewheel to couplethe distributor shaft and the planetary gear train, and a lockingelement operably connected with the freewheel and movable in dependenceon the rotational speed difference between a locked position in whichthe freewheel is locked and an open position in which the distributorshaft is able to freely rotate in relation to the planetary gear trainto a predetermined relative rotational direction.